A breast pump system comprises a breast pump acting as an expression unit, and an operating unit to operate the breast pump. The expression unit has a funnel which receives the breast and a receptacle in which the expressed milk is collected. The operating unit comprises a vacuum pump driven by a motor. In use, the vacuum pump applies a vacuum to the breast which enables milk to be expressed. The vacuum is applied to the breast either directly or via a membrane located in the expression unit which deforms and so causes a vacuum to be generated in the funnel.
It is known to provide a breast pump system in which a cyclical pressure differential vacuum is applied to the breast. This is achieved by the vacuum pump generating a vacuum exerted on the breast. Thereafter, the pressure from the vacuum is released by the use of a solenoid valve which temporarily opens. After the vacuum has been released, the solenoid valve closes so that the vacuum pump can build up a vacuum again. The opening and closing of the solenoid valve is repeated such that the breast pump system uses a cyclic pressure profile in order to express milk from the breast.
The solenoid valve of the breast pump system is configured to be closed when a vacuum is applied to the breast. However, if the solenoid valve fails to open so as to release the vacuum, the pumping of the vacuum pump will continue to build up the vacuum pressure to a maximum vacuum pressure allowed by the breast pump system. This maximum vacuum pressure is normally limited by an over-vacuum valve.
DE 3820211 A1 discloses a method and device for limiting the reduced pressure of an aspirator working with an aspiration phase and a venting phase.
U.S. Pat. No. 5,795,328 A discloses a vacuum system and a method of operating a vacuum system. The system comprises a vacuum line, a pump, a pump motor, a pressure adjusting assembly, a pressure sensor and a control means. The pump is connected to the vacuum line for drawing fluids therefrom, and the motor is connected to the pump to drive the pump. The pressure adjusting assembly is connected to the vacuum line for conducting fluid into that line at an adjustable rate. The pressure sensor is connected to the vacuum line to generate a pressure signal representing the pressure in the vacuum line. The control means receives the pressure signal and generates a system control signal that is used to adjust the pressure in the vacuum line.